The present invention is directed to a method of measuring physical characteristics, particularly but not exclusively strain, displacement and temperature, and in particular to such a method based on the measurement and analysis of the Brillouin scattering spectrum. The present invention is further directed to a fiber optic sensing configuration for use in measuring such physical characteristics.
When laser light pulses are propagated down an optical fiber, light is backscattered due to changes in density and composition, as well as molecular and bulk vibrations within the fiber material. That backscattered light includes Rayleigh, Brillouin and Raman backscattered components. The Raman backscattered light, caused by thermally induced molecular vibrations, can be used to obtain information on temperature distributions along the fiber. Thus, that technique has been demonstrated for using optical fibers as sensors for leakage detection in pipelines and underground storage vessels, for example.
Brillouin scattering results from scattering of light by sound waves, which produce a periodic modulation in the fiber""s index of refraction. That phenomenon is measured by the Brillouin frequency shift, given by the formula,
F=2nV/Lxe2x80x83xe2x80x83(1)
where F=the Brillouin frequency, n=index of refraction of the fiber, V=velocity of the light wave in the fiber and L=wavelength of the incident light in the fiber. Thus the application of mechanical strain and/or temperature to the fiber results in changes in xe2x80x9cFxe2x80x9d. The use of Brillouin loss spectrum analysis to measure strain and temperature with single mode optical fibers is superior to using the Brillouin xe2x80x9cgainxe2x80x9d technique, since it has been shown that the xe2x80x9clossxe2x80x9d method can be applied over longer fiber distances.
To obtain both temperature and strain, measurements of the Brillouin power as well as the Brillouin frequency shift (F) are required. The measured Brillouin strain in an optical fiber contains components associated with the fiber""s temperature (reflecting both ambient temperature and that of the structure to which it is attached) and the mechanical strain applied to the structure (to which the fiber is attached), given by the following equation,
E(x)=Et(x)+Em(x)xe2x80x83xe2x80x83(2)
where E (x) is the measured Brillouin total strain as obtained from measuring the Brillouin frequency shift (F) at any location xe2x80x9cxxe2x80x9d along the fiber, Et is the thermal strain component and Em is the mechanical applied strain. Thus the determination of Et allows one to calculate the mechanical strain knowing E(x).
It is known in the art to use the Brillouin frequency shift to measure optical fiber distortion, temperature along a fiber or both temperature and distortion. The use of a single laser light source to also measure temperature and distortion is also known. Limited applications of the Brillouin method utilizing buried optical fibers are disclosed in the prior art, purporting to measure earth sloping, distortion of the ground between fixed points and the motion of embedded weights attached to an optical fiber. A method of measuring a single optical fiber""s distortion between two fixed points using Brillouin scattering is also known.
Brillouin instruments have been developed to measure temperature distributions over long distances using single mode optical fiber, where the fiber runs, e.g., along the bottom of a lake. It has also been shown that a Brillouin instrument can measure the concrete curing temperature distributions in a dam. One commercial Brillouin instrument using a single DFB light source is known, but it is limited to a strain accuracy of xc2x1100 to 300 microstrain, or 0.01% (1 microstrain=10xe2x88x926 mm/mm or in/in). Such strain accuracies are not suitable for applications to bridges and pipelines, for example, where maximum operating strains are of the order of 100 microstrain.
The typical Brillouin instrument system used to measure strains and temperatures, shown schematically in FIG. 1 as 100, can incorporate one or two light sources. To achieve better strain measuring accuracies, it is known to use two separate xe2x80x9cfrequency tunablexe2x80x9d laser light sources 102, 104 operating at about 1320 nm wavelength. One laser 102 acts as a pump laser, while the other laser 104 serves as the probe laser which sends optical pulses down the fiber 106 to interact with the counter propagating laser lightwave pumped into the fiber 106 from its opposite end. Each laser 102, 104 is in optical communication with the fiber 106 through a polarization controller 108, 110. In addition, a pulse generator 112 controls a modulator 114 to modulate the light from the pump laser 102 to form pulses. A circulator 116 diverts light from the fiber 106 into a signal detector 118, whose output is applied to an oscilloscope 120. In addition, the output of a detector 122 is applied to a spectrum analyzer 124, whose output is applied to an oscilloscope 126. The outputs of both of the oscilloscopes 120, 126 are analyzed in a data acquisition system 128. The system 100 operates in a manner which will be familiar to those skilled in the art.
It is well known to those skilled in the Brillouin technology that both lasers can be located at one end of the fiber, providing the other end has a mirror (or some other reflective optical element) to reflect the laser wavelengths. A possible configuration to perform that task is shown in FIG. 2 as 200, in which the light from the lasers 102, 104 is applied to the same end of a Brillouin fiber sensor 206. In FIG. 2, reference numerals 230 and 232 designate couplers and sections of single-mode optical fiber, respectively, while those reference numerals which are common to FIGS. 1 and 2 have the same significance in both of those figures. In addition to the laser light sources, other instrumentation components include, but are not limited to, a pulse generator, a spectral analyzer and a signal detection system. The theoretical description of how that Brillouin loss technique works is known in the art.
It is known that a Brillouin system can be used to measure strain over optical fiber distances exceeding 50 km. It is also known that a Brillouin system can achieve a strain resolution of typically as low as xc2x120 microstrain, over gage lengths as small as 10xcx9c15 cm, and can measure temperature changes as low as xc2x11xc2x0 C. Such measurements, based on the system shown in FIG. 1, obtain information on the Brillouin frequency shift and the Brillouin loss spectrum, which combine to yield simultaneous measurements of the strain and temperature over the selected gage length.
Applications of the Brillouin loss technique, as described in the published literature, are limited to laboratory materials and small test structural elements such as a steel beam and concrete beams. None of the published documents employ, or describe, in their experiments or test cases, how to apply the Brillouin loss technology to large structures such as pipelines, dams, buildings or bridges, for example. No data or design concepts on large structural applications have been reported in these documents or their related references contained in their publications. No mention is ever made of the potential use of multiple Brillouin sensors operating off a single fiber optic backbone.
It will be apparent from the above that a need exists in the art to overcome the above-noted deficiencies in the art. It is therefore a primary object of the invention to apply Brillouin loss technology to large structures.
It is another object of the invention to apply Brillouin loss technology to measure physical characteristics such as strain or displacement over large structures and also, optionally, to determine the location of the strain or displacement and to compensate for temperature.
To achieve the above and other objects, the present invention is directed to the application of a fiber optic sensing system based on the measurement and analysis of the Brillouin scattering spectrum to measure, e.g., strain (or displacement) and temperature distributions remotely, over long distances, with a controllable gage length, at any location along an optical array of fibers (including the special case of a single fiber) which have been attached to any large structure such as a pipeline, dam, building or bridge or any other structural configuration. In particular, the attachment of optical fibers (single mode fiber is the preferred fiber material) to the surface of a structure, or embedding the fibers in a structure (such as a concrete casting for example), and measuring the Brillouin scattering spectrum (the preferred method employs the xe2x80x9clossxe2x80x9d spectrum) allows one to determine simultaneously the state of strain (or displacement) and temperature averaged over specific xe2x80x9cgage lengthsxe2x80x9d, at selected locations along the fiber array. A Brillouin Displacement Sensor (BDS) is one embodiment of that fiber optic sensor system.
The advantages of using Multiple Brillouin Sensor Arrays (MBSA) are: that system allows for sensing redundancy in that if one or more sensors fail, there are other sensors still active; multiple sensors permit the splitting off of the sensors to different parts of a large structure, thus making sensor routing relatively easy compared to using a single fiber sensor; the sensors can be attached to a structure in such as way as to discriminate between temperature and mechanical strain by leaving some sensors unbonded to the structure to obtain thermal response, while the attached or bonded sensors measure combined thermal and mechanical straining, as denoted by Eq.2.
Brillouin scattering spectrum analysis can be implemented using multiple Brillouin fiber optic sensors to measure strain, displacement and temperature on structures. The Brillouin sensors are routed off a common fiber optic backbone to various parts of the structure in which the backbone itself can be used as a Brillouin sensor. As a displacement measuring system, the array of Brillouin sensors can be attached at fixed points, and optically coupled to a fiber backbone to measure discrete displacements at a number of locations on a structure or foundation to yield information on such aspects as growth of cracks and fissures and ground settling effects on structures, for example. Application embodiments highlighted include, but are not limited to, pipelines, bridges and ground movements, as examples. Separation of temperature strains from mechanical strains is demonstrated using multiple sensors, and incorporating unbonded sensor gage lengths. The use of coarse and fine scan pulse widths is demonstrated to permit the interrogation over long distances (such as pipelines) to isolate regions of the structure (in shorter times using the coarse scan mode) where more detailed evaluation of the strain field is required using shorter gage length pulses.
In an illustrative embodiment, the multiple Brillouin sensors can be optically coupled to a common optical fiber backbone using a Brillouin instrument for generating the light waves and sensing signals, based on one of the instruments shown in FIGS. 1 and 2. One can design the modulated pulse width to achieve a desired sensor gage resolution for any of the Brillouin sensors of that embodiment. A particular application of the MBSA concept is for a pipeline consisting of multiple pipe sections, each of which has a Brillouin sensor (of any length) bonded or attached to the exterior surface of the pipe wall. Interrogating the multiple Brillouin sensors provides distributions of temperature and strain along the pipe sections. If one employs a single Brillouin fiber optic sensor, a methodology for measuring strain and temperature distributions along the pipeline is disclosed for a spiral wrap technique, which may be used for different sections of the pipeline.
Another concept of employing the MBSA system is disclosed in which displacement distributions are made using multiple fixed points to which the sections of the optical fiber are attached. Each Brillouin sensor mounted between two fixed points can constitute a portion of a continuous optical fiber (as shown) or can be configured as separate Brillouin optical fiber sensors, optically connected to a common fiber backbone. The use of optical fiber loops to allow for pre-tension of the sensing fiber (which may be employed in either the form of a continuous fiber, or as separate fiber sensors) permits the measurement of contraction (ie: shortening of the sensor by virtue of the fixed points moving closer together) or elongation (ie: extension of the fiber sensor by virtue of the fixed points moving away from each other). Applications of the Multiple Brillouin Displacement Sensor (MBDS) system can include, but are not limited to, measuring ground displacements associated with dams for example, movement of foundations, growth of ground cracks/fissures in seismic fault areas, and long term erosion. The displacement range and Brillouin sensor sensitivity to very small movements can be designed according to the Brillouin instrument used, which has shown a strain accuracy of xc2x120 microstrain. That corresponds to a displacement accuracy of about 0.4 mm. for a 20 meter long gage length. One can assess the range of structural displacements one can achieve for different Brillouin strain measurements and Brillouin sensor gage lengths employed.
Applications of single (and multiple) Brillouin strain sensors to bridges, are disclosed. It will be shown below how the sensor can be attached to a number of girders by routing them down the span of the bridge, and connecting at one fiber end. As in other cases described above, if two fiber ends are required, depending on the instrumentation used, then the return fiber can be routed back to the instrument. Helically wound Brillouin fibers can also be bonded or attached to concrete or steel bridge support columns, to measure hoop (circumferential) expansion due to material corrosion, as another embodiment.